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Introduction: Photodynamic diagnosis (PDD) as a method for differentiating between malignant tumors and normal tissue attracts a lot of attention. The 5-aminolevulinic acid (5-ALA), a naturally occurring amino acid with no fluorescent properties, metabolizes into protoporphyrin IX(PpIX), a fluorescent substance that gives off 630nm red absorption light when irradiated with blue light of 405nm. PpIX also has the property to accumulate in carcinoma cells. PDD using 5-ALA is currently put to practical use in the diagnosis of malignant brain tumors. Although it’s application to colorectal cancer in animal models has been reported, there is little information or pathological evidence regarding the uptake of tumors in humans. We tried applying PDD of 5-ALA using resected specimens from patients with colorectal cancer. Method: 5-ALA (Cosmo bio company) was dissolved in saline with a ratio 20mg/kg and taken orally one hour before the start of anesthesia. Following the specimen extraction, we conducted our fluorescence observation using a wavelength of 375-440nm. 630nm red absorption light was visible with the naked eye through a filter that allowed us to compare and confirm the accuracy of the original diagnosis. Results: 5-ALA was given to 12 patients in total, there were 1 A-colon cancers, 1 T-colon cancers, 3 D-colon cancers, 5 S-colon cancers, 2 rectal cancers. By observing the fluorescence, colorectal cancer could be confirmed macroscopically in the resected specimens, confirming the accuracy of the histopathological diagnosis. One of three patients with metastases to lymph nodes was identified fluorescence. Nine patients without metastases to lymph nodes confirmed no fluorescence. As a preliminary study, two patients were measured spectrum of tumors and lymph nodes. Conclusion: It seems that PDD using 5-ALA was significant in colorectal cancer histopathological diagnosis.